The term "laser" is an acronym for "light amplification by stimulated emission of radiation." Lasers produce beams of coherent monochromatic light in both the visible and other portions of the electromagnetic spectrum. Laser beams have extremely high energy resulting from their single wavelength and frequency. Because of their high energy, laser beams are used in a number of industrial applications for cutting difficult materials, or items that can or must be cut or scribed in extremely controlled patterns.
Accordingly, because the manufacture of semiconductor devices requires relatively precise tolerances and patterns on an extremely small scale, attempts have been made to use laser light to pattern, cut, or otherwise treat semiconductor materials during the manufacturing process. To date, these efforts have met with little success. The main problems appear to be twofold: first, the power of the laser light can damage the devices being manufactured. Second, the reaction between the laser light and the material being treated, particularly material being cut, produces byproducts that remain on the surface of the structure being manufactured and must be removed later. If removal techniques are unavailable or unsatisfactory, such byproducts can degrade or even ruin the resulting devices.
Light-emitting diodes (LEDs) are semiconductor devices that emit light, including visible light, when a potential difference is applied across a p-n junction structure. There are a number of ways to make light-emitting diodes and many associated structures, but these are generally well known, and the invention that will be described herein applies to most or all of them. Thus, they will not be discussed in detail hereafter except as necessary to explain the invention. By way of example, and not of limitation, Chapters 12-14 of Sze, Physics of Semiconductor Devices, 2d ed. (1981), gives a good explanation of a variety of photonic devices, including LEDs.
As is known to those familiar with semiconductor materials and the devices made from those materials, however, the color of light that can be produced by a light-emitting diode is generally limited by the characteristics of the semiconductor material, and most significantly by the bandgap. The bandgap represents the energy transition between the valence band and conduction band of individual atoms. In accordance with well-understood quantum mechanical principles, transitions between the valence band and the conduction band are limited to the precise bandgap, or to definite intermediate states related to the bandgap that are likewise characteristic of the material and its dopants or impurities.
Stated more simply, the bandgap of a material limits the color of light that it can emit based on the bandgap transitions that generate such light.
As is further well known to those familiar with semiconductor materials, light, and their interaction, the energy of a photon emitted by a transition is related to its frequency through the formula E=h.nu., where "E" is the energy of the photon, "h" is Planck's constant, and ".nu." is the frequency. Thus, the bandgap's energy width, usually expressed in electron volts ("eV"), limits the photons it can produce to certain energies and therefore certain frequencies.
In turn, the frequency of a photon is inversely related to its wavelength according to the formula .lambda.=c/.nu., where "c" is the speed of light and ".lambda." is the resulting wavelength.
Because of these limitations, materials with smaller bandgaps can only produce longer wavelength, lower frequency photons, which fall towards the red (770-622 nm), orange (622-597 nm), and yellow (597-577 nm) portions of the visible spectrum. Light tends to begin to have a purer green color at about 525 nanometers, and thus a bandgap of approximately 2.36 eV is required in a semiconductor material before it can produce such photons. Similarly, a truer blue emission has a wavelength on the order of 470 nanometers and thus requires a bandgap of 2.64 eV or greater. It will be understood, of course, that the assignment of particular wavelengths to particular colors or to the boundaries between colors is somewhat arbitrary and should be taken as illustrative rather than absolute.
There are only a few semiconductor materials that have the appropriate bandgaps and can produce such light. Two such candidate materials are silicon carbide (2.86 eV for .alpha.-SiC) and gallium nitride (3.36 eV).
The theoretical properties of silicon carbide have been recognized for some years, but only in the last decade has sufficient progress been made in techniques for bulk crystal growth, epitaxial growth, and device manufacture, much of it by the common assignee of this invention, to produce workable devices from silicon carbide. Silicon carbide has an extremely high melting point (about 2830.degree. C.), is physically extremely hard (often used as an abrasive), and crystallizes in over 150 polytypes, most of which are separated by relatively small thermodynamic differences.
Gallium nitride and its related Group III nitrides (i.e., Group III of the periodic table) are other candidate materials, but to date no satisfactory method has been found for producing large bulk single crystals of gallium nitride or other Group III nitrides that could serve as appropriate device substrates. Thus, gallium nitride devices have typically been formed on sapphire (Al.sub.2 O.sub.3), and more recently on silicon carbide.
Most of the successful efforts so far in producing LEDs that emit in the blue region of the spectrum have used a "mesa"-type structure. The term "mesa" generally defines a structure in which the base or substrate of a device has a larger cross-sectional area than the active area, which is typically formed by two or more epitaxial layers on the substrate. The conventional mesa structure provides a physical separation between active regions when a plurality of devices are manufactured on a single wafer, which presently is the most common method of producing such devices in large quantities. The space between mesas provides a portion at which the devices can be mechanically separated. Typically, mechanical separation is carried out with a tool of some sort capable of working with such hard materials, for example a diamond saw, or as more recently set forth in copending, commonly assigned application Ser. No. 08/290,458, filed Aug. 15, 1994, an electrodischarge machine ("EDM"). Typically, EDM can be used to separate large bulk single crystals into substrate wafers, and a diamond saw is used to cut individual die from a wafer once devices have been formed on it.
In some circumstances, however, the mesa structure limits the efficiency of the manufacturing process and the resulting devices. The mesa structure lowers manufacturing efficiency because it tends to require a fairly large percentage of the area of a wafer in the form of the separation between individual mesas. Thus, to the extent that the separation between mesas can be reduced, the percentage area of the wafer that carries active device structures can be correspondingly increased. To date, however, the diamond saw remains a limiting factor in the extent to which mesa separation can be reduced.
Diamond saw-cutting also tends to produce relatively smooth finishes on the individual LED die, and these smooth surfaces tend to encourage internal reflection of light from an LED, rather than external emission. In this regard, it will be understood by those familiar with the devices, that the light is emitted from the junction of the device, and must travel to and leave the edges of the device in order to be visible. Thus, to the extent the light is internally reflected, it reduces the external quantum efficiency of the resulting device.
One technique for increasing the external efficiency is set forth in copending, commonly assigned application Ser. No. 08/081,688, filed Jun. 23, 1993, which describes an extended epitaxial layer for taking advantage of certain optical considerations in increasing the external efficiency of such diodes.
When attempting to use laser cutting with materials such as silicon carbide or gallium nitride, however, it has been found that the laser's action creates both expected and unexpected problems. The expected problems include unwanted byproducts and damage to the devices being formed. The unexpected problems arise from a disadvantage that is particularly characteristic of silicon carbide. Specifically, in other materials, damage to the device, and particularly to its crystal structure, can change the intensity of the color produced by the material, but not the color itself. In contrast, if certain types of damages are created in silicon carbide, particularly point defects, they tend to form recombination centers that change the visible emission of the resulting diode from blue to green. Thus, although green LEDs in silicon carbide are desirable when they can be produced in a controlled fashion (see e.g., copending application Ser. No. 08/290,020, filed Aug. 12, 1994), producing a green LED is quite disadvantageous when a blue-emitting LED is desired.
To date, there has been no satisfactory resolution of these problems with respect to laser cutting in general and laser cutting of silicon carbide and gallium nitride in particular.